In electrophotographic image forming apparatuses such as copiers, printers and facsimiles, a developer has an important role in finally visualizing an image and technical development thereof is very actively made. Particularly, a mainstream dry two-component developer is formed of a particulate magnet called a carrier and a particulate resin including a colorant called a toner.
The carrier is a magnetic powder including a core material and a thin resin layer formed thereon for the purpose of controlling resistivity, imparting chargeability and improving durability, and is prepared by coating a coating liquid on the core material, burning and sifting.
Japanese published unexamined application No. 2010-282168 discloses using a burner such as an electric oven and rotary kiln in a burning process of preparing a carrier.
However, these burners heat heaters to heat air, and the heated air heats the particulate carrier. The particulate carrier is indirectly heated through air having low heat conductivity. Therefore, the energy efficiency is low and a specific energy consumption (an energy requited to produce a unit weight [kWh/kg]) is large.
In order to solve this problem, using a high-frequency induction heating in the burning process of preparing a carrier is discussed.
The high-frequency induction heating is a method of heating metals, using an electromagnetic induction phenomenon. Two iron losses called an eddy-current loss and a hysteresis loss heat the conductive core material of a carrier.
The eddy-current loss is an iron loss generating a Joule heat because an eddy current caused by a magnetic line generated from a conductive line a high-frequency current flows through flows through the core material having electrical resistance.
The hysteresis loss is an iron loss generating a heat when a magnetic flux generated in the core material causes a hysteresis phenomenon when a high-frequency current flows through a coil.
In the high-frequency induction heating, the core material is heated by a heat generated from each of the two iron losses to dry a solvent remaining in a coated film and heat a resin. Therefore, each core material can directly be heated not through a medium such as air, and the high-frequency induction heating is expected to be a burning method having a very small specific energy consumption.
However, a voltage, a current and a frequency are thought limited in using a high-frequency induction heater. Hereinafter, the limitation is explained.
FIG. 16 is a circuit diagram of a resonance LCR circuit included in a high-frequency induction heater.
In the resonance LCR circuit in FIG. 16, the following relationship is satisfied:Vc=Q×V wherein Vc represents a voltage resistance of a condenser and V represents a source voltage.
Further, the following relationship is satisfied as well:
  Q  =            1      R        ⁢                  L        C            wherein R represents a resistance, L represents an inductance and C represents a condenser capacity.
When “Q” formula is substituted in “Vc” formula, the following relationship is satisfied:
  Vc  =            1      R        ⁢                  L        C              ⁢    V  
Therefore, the source voltage V is represented by the following formula:V=Vc·R·(C/L)0.5   (1)wherein V represents the source voltage, Vc represents the voltage resistance of a condenser, R represents the resistance, C represents the condenser capacity, and L represents the inductance.
In the induction heating, the larger an intensity of a magnetic field formed of a current flowing a coil, the larger a calorie supplied to a carrier per a unit time, and the productivity is increased. The intensity of a magnetic field is proportional to the number of coil turns per a unit length and a current. The source voltage V needs increasing to increase a current flowing through a circuit having the fixed resistance R and the fixed inductance L. From the formula (1), when a carrier having a fixed resistance R is burned with a coil having a fixed inductance L, the voltage resistance of a condenser and the condenser capacity need increasing to increase the source voltage V. However, the source voltage V is limited because the voltage resistance and the capacity of a condenser are both limited.
Typically, a current is represented by the following formula:I0=V0/R0 wherein I0 represents a circuit current, V0 represents a source voltage and R0 represents a circuit resistance.
Therefore, I0 is thought limited when V0 is limited.
A current frequency in the resonance circuit is represented by the following formula:f=1/2π[1/(L·C)]0.5   (2)wherein f represents a frequency, L represents the inductance and C represents the condenser capacity.
The higher the frequency of a current, the lower a depth of penetration of an induction current generated in a conductive material to be heated. Therefore, the higher the frequency of a current, the more efficiently a small particulate carrier can be heated. However, from the formula (2), when a carrier is burned with a coil having a fixed inductance L, the condenser capacity C must be decreased to increase the current frequency f, but the condenser capacity C is limited. Further, as the formula (1) shows, when the condenser capacity C decreases, the source voltage V decreases as well, which is not necessarily be effective to increase burning efficiency, and the current frequency f is limited as well. In addition, the high-frequency induction heater has a fixed rated frequency, a high-frequency oscillator included therein cannot be oscillated with a frequency out of the fixed rated frequency.
From the formula (2), it is obvious that decreasing the inductance L enables the source voltage V and the current frequency f to increase without receiving limit of the condenser capacity C. Effective means of decreasing the inductance L include decreasing the number of coil turns. However, the intensity of a magnetic field is proportional to the number of coil turns per a unit length and a current. Therefore, when the number of coil turns is decreased while the number of coil turns per a unit length is maintained, the coil has a short length and a heating area narrows. When the number of coil turns is decreased while the length of the coil is maintained, an electric power required to burn a specific amount of a carrier increases, and a specific energy consumption is thought to deteriorate.
In this manner, the source voltage V, a circuit current I and the frequency f are limited in burning a carrier by a high-frequency induction heater, the oscillating capability of the high-frequency oscillator is not fully drawn, resulting in poor specific energy consumption.
Because of these reasons, a need exist for a method of preparing carrier for electrophotography, which has high productivity and low specific energy consumption.